In the field of dentistry, it is necessary to measure the mobility or angular displacement of a tooth or dental implant to predict the long term integrity of the tooth or implant. In particular, in preparation for securing a crown to an implanted fixture, the integrity of the implant in the patient's jaw must be tested. Generally, an opening is created in the patient's jaw and a setting for the fixture is inserted into the opening. The setting materials have been developed to be highly compatible with human bone, whereby under ideal circumstances, the patient's bone tissue surrounding the setting osseointegrates and effectively incorporates the setting into the jaw structure. After a reasonable healing period, the integrity of the setting in the jaw is evaluated. If there is any movement of the setting, it is concluded that the setting has not adapted sufficiently to the jaw to support a dental implant. Similarly, for an existing tooth, it may be concluded that the tooth is not sufficiently rooted in the mouth structure to sustain normal use without additional support in the form of bridges, etc.
Available tooth displacement sensors are detailed in the article entitled "Review of Methods for Measuring Tooth Mobility" by Dr. Samuel Yankell, which was published in 1988 in the Compendium of Continuing Education, Dental Supplement No. 12, pages S428-S432. As detailed therein, early sensors comprised calipers which recorded the physical displacement of the tooth in response to pressure applied thereto. Assessing the exact amount of pressure to which the tooth is subjected is difficult in such a system. Furthermore, the caliper-based systems were inexact given the fact that the calipers themselves often move in response to the applied pressure, thereby influencing the measurements. Later sensors have been developed to measure tooth mobility without the application of an external force to the tooth and measuring tool. Eddy current displacement sensors, holographic interferometers, and stereophotography systems have been used to measure tooth mobility without the measuring tool physically contacting the tooth and thereby, to some degree, physically influencing the tooth movement. Each of the foregoing systems is able to take a "picture" of the tooth under various load conditions, which conditions simulate either expected dental procedures or normal usage to which the tooth or fixture will be subjected. An analysis of the "pictures" provides a profile of the tooth or fixture mobility. A disadvantage of the latter methods of measurement is the need to rely on an external standard by which the movement observed in successive "pictures" is correlated to actual displacement in response to a load.
In unrelated technologies, tilt sensors have been developed utilizing several basic approaches. Low gravity accelerometers utilizing force balance spring mechanisms provide highly accurate tilt angle measurement; however, such systems are large in size incorporating high cost, complex electromechanical transducers. Though appropriate for large scale tilt or attitude measurements, such as for aeronautic applications, such sensors cannot be scaled to be workable in smaller dimensions.
Another category of angular measurement devices rely on the use of a variable capacitor or variable resistor positioned in an enclosure which is partially filled with either a dielectric or a conductive fluid. Upon angular displacement of the enclosure, the movement of the air bubble above the fluid is detected by measurement of the change in the electrical capacitance or resistance.
An angular displacement sensor is provided in U.S. Pat. No. 3,839,904 of Stripling, et al, which is entitled "Magnetic Fluid Level Detector And Vibration Transducer". The Stripling, et al sensor provides a magnetic fluid sensing element having a primary coil and two secondary coils disposed about a vial which is partially filled with fluid. A single magnetic flux path passes through the primary and the two secondary coils, with a portion of the flux path being in the air gap about the fluid in the vial. Angular displacement of the vial results in displacement of the fluid and a consequent change in the magnetic field, resulting in an output voltage indicative of the displacement. Such a sensor must be relatively large in size in order to compensate for the large presence of an air gap in conjunction with sufficient ferromagnetic fluid for the desired magnetic permeability.
U.S. Pat. No. 4,676,103 of Nakajima provides "Acceleration or Inclination Sensors" which operate in a similar manner to the above-noted category of systems. The Nakajima sensors effectively utilize a bubble of magnetic fluid which is displaced in response to angular displacement of its housing.
Some disadvantages of the sensors which rely in part on the air bubble or air gap in the sensor enclosure include variability in readings due to ambient temperature fluctuations, relatively low output signal levels due to the fact that air is a high reluctance path for magnetic flow, and lack of scalability to ideal dimensions given the need to increase fluid amounts to compensate for the high reluctance air path.
What is desired, therefore, is a system which provides high output signals, representative of angular displacement, in a small-scale, temperature stable sensor.
It is, therefore, an objective of the present invention to provide a system and method for measuring angular displacement of a body on a small scale.
It is another objective of the invention to provide a system and method for measuring angular displacement of a body without requiring external calibration.
It is still another objective of the invention to provide a system and method by which angular displacement of a body can be measured without applying force to the measurement tool itself and thereby affecting the results of the measurement.
Yet another objective of the invention is to provide an angular displacement sensor which can effectively provide high output signals representative of angular displacement in a small-scale, temperature-stable sensor.